Conventional television signal distribution provides approximately 500 vertical lines of resolution by transmitting luminance and chrominance information on analog signals having bandwidths that are typically less than 5 megahertz (MHz). Conventional television sets receive the analog signals and modulate the operation of a display device, such as an electron gun or a liquid crystal display (LCD) panel, to present the luminance and chrominance information on a video screen as video to a viewer. This form of television is referred to as standard definition television (SDTV).
In the 1990's high definition television (HDTV) began to emerge. As the name implies, HDTV provides higher resolution than SDTV by providing more pixels, or lines, of information for presentation on a video screen. HDTV broadcasts have become common; major networks now broadcast programming using both conventional SDTV and HDTV. Additionally, high definition (HD) equipment, such as HD receivers and televisions, has become widely available at reasonable prices. In fact, many televisions available today are capable of presenting video in HD and standard definition (SD) modes. While some televisions include HD receivers, some rely on external receivers present in set-top boxes (STBs) to perform the necessary decoding and processing of HDTV signals. Such STBs receive and process HDTV signals and provide component outputs (e.g., Y, B-Y, and R-Y) or composite outputs to HDTVs. The HDTVs, in turn, receive the outputs from the STBs and present the video on the HD display screen.
One problem that has arisen with the existence of both SDTV and HDTV programming, as well as the presence of SD equipment and HD equipment, is the presentation of SDTV programming on HD displays.
Displaying SDTV information on SD equipment is well known. The television industry has adopted the approach of keeping the overall transmission system response of an SDTV signal as flat as possible up to the display, when processing horizontal and vertical video information is done to achieve the best image quality. That is, the receivers, mixers, amplifiers, including the entire broadcast chain and other system components are designed to provide as flat of a frequency response as possible. Conventionally, a color display is a cathode ray tube (CRT) with a color mask. In SDTV, the size of the color mask spot determines the size of the spot that is scanned across the screen when a video image is rendered. The size of the spot is matched to the frequency bandwidth of the video system. The spot size is kept large to prevent aliasing (i.e., the appearance of high frequency noise as low frequency terms) and to keep the displayed signal bright. In a sense, the television acts as a two dimensional filter of an image, which, in most cases, limits the frequency response of the system.
Displaying HDTV information on HD equipment is similar. Again, the display acts as a final two dimensional filter that limits the overall system response. HD equipment is designed to match the bandwidth of the HDTV signal.
While displaying SDTV and HDTV information on their respective equipment is known, the situation becomes more complex when displaying SDTV information on HD equipment. In this situation, the HD equipment has much more spatial bandwidth than does the SDTV signal. Thus, to display a SDTV signal on HD equipment, the SDTV signal must be upsampled. That is, the information in the SDTV signal must be extrapolated, or augmented.
Upsampling itself is a challenging, but known, task. However, upsampling is made more difficult by digital television operators who use many different SDTV resolutions when broadcasting SDTV signals. For example, a digital versatile disk (DVD) source outputs 720 pixels per line, while a digital television operator, such as DIRECTV®, may provide programming ranging from 352 pixels per line to 720 pixels per line. In such an arrangement, an upsampler designed for 720 pixel resolution will make 352 pixel information look poor on the HDTV display.
HD equipment providers, such as DIRECTV®, validate receiver designs prior to deploying those designs in the field. Conventionally, upsampling from SDTV to HDTV is tested through application of a test signal(s) at the beginning of the receiver filter chain. The output of the filter chain is then evaluated to determine upsampler performance. Conventional test signals include steps, pulses, and sweeps having responses that provide information as to the horizontal, vertical, and temporal resolution of the system.
However, typically, HD equipment, such as STBs, are highly integrated and, thus, the ability to isolate the upsamplers, which are typically the source of poor image quality, is limited, as the input to the upsampler is not exposed for test signal injection. For example, a conventional transmitter and receiver lineup includes a motion pictures expert group (MPEG) encoder and an MPEG decoder, each having filtering or prefiltering, that influence test results and prevent isolation and measurement of upsampler performance.